Radio antennae



March 12, 1957 B. B. JACOBSEN ETAL 2,785,398

RADIO ANTENNAE Filed July 3. 1952 Inventor B- 8. JACOBSEN D. L, THOMAS mww Atlorney United States Patent RADIO ANTENNAE Bent Bulow Jacobsen andDavid Lane Thomas, London, England, assignors to International StandardElectric Corporation, New York, N. Y.

Application July 3, 1952, Serial No. 297,088 Claims priority,application Great Britain July 9, 1951 4 Claims. (Cl. 343-781) Thisinvention relates to antenna arrangements of the type comprising aconcave mirror having a co-operating antenna located at or adjacent tothe focal point of the mirror. More particularly, it relates toarrangements of the said type in which the construction is such that oneor more reflecting surfaces obstruct in part the mouth of the mirror andreflect back thereto some of the energy which would otherwise beradiated by the mirror into space. Such reflection-back effects theimpedance matching of the co-operating antenna and renders it moredifiicult to avoid standing waves, variable with frequency, in theantenna feeder, this effect being of particular importance in the caseof antenna arrangements for wide-band systems such as television ormulti-channel links. It is accordingly an object of the presentinvention to provide an antenna arrangement of the type described above,adapted for minimising the effect of reflection-back from surfacesobstructing radiation from the mouth of the mirror.

According to its most general aspect the invention resides in a radioantenna arrangement comprising a concave mirror, a co -operating antennalocated at or adjacent to the focal point of said mirror, and at leastone member so located as to obstruct radiation from said mirror,characterised in this, that said obstructing member is so formed as toreduce the effect of reflection back to the mirror of radiationtherefrom intercepted by said member.

The invention is illustrated and explained in connection with theaccompanying drawings, in which:

Fig. 1A illustrates the undesired reflection phenomenon in an antennaarrangement which does not incorporate the present invention; and

Fig. 18 illustrates an embodiment of the invention.

in these figures the same reference numerals are used to designatecorresponding parts. Only those parts of the antenna arrangements areshown which are essential to the understanding of the invention.

One particular antenna arrangement in which the said refiection-backeffect has been found to be of importance is illustrated in Fig. 1A ofthe accompanying drawing and comprises a paraboloidal mirror 1 having atits focus "a co-operating antenna in the form of a horn 2 which iscoupled to an energy translating device (not shown) such as atransmitter or receiver by means of an H-bend 3 followed by a linearwave guide 4 of rectangular crosssection, which wave guide lies radiallyacross the mouth of the mirror with one wall of the guide parallel tobut slightly spaced out from the aperture of the mirror. In thisarrangement energy projected from the mirror parallel to the generatingaxes impinges normally on the wall 5 of the linear guide and isreflected therefrom back to the mirror surface and thence to the hornantenna along paths a and b for example. Since the total reflection pathfrom any point on the reflecting wave guide surface to the horn antennavia the mirror surface is approximately constant, the' contributionsfrom difierent points 2,785,398 Patented Mar. 12, 1957' along the lengthof the .guide are approximately cophasal, the total being of appreciablemagnitude.

One method of reducing the reflection-back is to provide an additionalreflecting surface the reflection from which is equal in magnitude butopposite in phase to that just mentioned. This may be done, in oneembodiment of the invention, by adding an auxiliary reflecting membyextending the wave guide linearly beyond its junction with the l-l-ben-dcompletely across the mouth of the mirror by means of a dummy portion ofwave guide as illustrated at 6 in Fig. 1B, the dummy portion having aflat reflecting surface 7 facing the mirror, and of the same dimensionsas the wave-guide proper, but displaced nearer the mirror by a distanceof one quarter of the mean operating wavelength. This displacement isconveniently obtained by the superposition of a conducting strip 8 onthe wall of the dummy portion of waveguide, this strip being of the samewidth as the waveguide wall, and one quarter wavelength in depth. Sincethe reflecting surface of the waveguide proper and the spacing strip areequal and symmetrically located with respect. to the antenna horn, veryeffective cancellation of the reflected back energies is obtained. Thedummy portion of wave guide may also have a mechanical advantage in thatit may be used as a member for improving the rigidity of thehornand-mirror assembly.

The same principle may be applied in other ways. For example, the dummywaveguide may be dispensed with, and a quarter wave strip applied over asufficient length of the wave-guide proper to give cancellation betweenthe reflection from the strip and that from the uncovered part of thewave-guide. Moreover, it is not necessary that the spacing strip be somuch as one quarter wavelength thick; a thinner strip may be usedprovided that the mounting means is such that the proper spacing outfrom the reflecting surface, or from the plane thereof, is obtained.Using a thin strip, the mounting mean may be made adjustable, and theshape of the strip need not be limited to one of the same width as theguide Wall, so long as the area of the strip is such that the totalreflection therefrom balances the remaining reflection. Moreover, thestrip may be divided into two efiective halves spaced respectively onequarter wavelength forward and one quarter wavelength backward from theplane of the offending surface, such disposition giving compensationover a very wide band.

Instead of using the principle of reflection-cancellation to remove theunwanted effect, the construction may be made such that the totalreflection from an obstructing surface is small ab initio. For example,the obstructing surface may be coated with a layer of material ofsurface resistance such that the high frequency energy impingent thereonis absorbed instead of being reflected. Alternatively, the surface maybe so shaped that components of reflection from different points add inrandom phase instead of cophasally for example by using a curved surfaceinstead of a flat one for the outer wall of the guide, or by curving theaxis of the guide, in a plane through the axis of generation of theparaboloid.

While the principles of the invention have been described above inconnection with specific embodiments, and particular modificationsthereof, it is to be clearly understood that this description is made byway of eX ample and not as a limitation on the scope of the invention.

What we claim is:

1. A radio antenna arrangement comprising a paraboloidal mirror and aco-operating horn antenna located at or adjacent to the focal point ofsaid mirror, a rectangular metal wave guide coupling said antenna to anenergy translating device, a major part of said wave guide being linearand located in front of said mirror with one wall of greases the guideparallel to the aperture plane of the mirror, the surface of which Wallreflects energy radiated thereon by said mirror, and a collinearextension of said coupling wave guide beyond the junction with the hornand extending in front of said mirror, the external surface of saidextension facing said mirror having a flat radiationreflecting surfacein a plane parallel to that containing the reflecting surface of saidone Wall but displaced therefrom by a distance of one quarter of theoperating wavelength, whereby reflection from part of the surface ofsaid dummy Wave guide cancels the reflection from a corresponding partof the surface of said one wall.

2. A radio antenna arrangement comprising a mirror, a cooperatingantenna located in front of said mirror and adapted to radiate energy inthe direction of said mirror, transmission means for feeding saidantenna, a portion of said transmission means being located in front ofsaid mirror and having a surface which reflects energy radiated thereonby said mirror, a reflecting member forming a collinear extension withsaid transmission means extending in front of said mirror, thereflecting member having a radiation-reflecting surface, spaced at adifferent distance from said mirror than said transmission means tocancel the effects of reflection from a corresponding part of saidtransmission means.

3. The arrangement according to claim 2 wherein said collinear extensionis displaced from said transmission line a quarter wavelength of theoperating frequency in the direction of said mirror.

4. A radio antenna system comprising a curved mirror, an antennapositioned in front of said mirror substantially at its focus, atransmission means connected to said antenna and having a reflectingsurface positioned in front of said mirror, whereby disturbingreflections back to said antenna are produced, and means forsubstantially cancelling the elfects of these disturbing reflections,comprising a reflecting means having substantially the same refleetingproperties as said reflecting surface, positioned in front of saidmirror and extending in a direction substantially symmetrically withrespect to the transmission means, at a spacing from said mirrordiffering from the spacing of said transmission means by substantiallythe equivalent of a quarter wave length at the operating frequency.

References Cited in the file of this patent UNlTED STATES PATENTS2,429,60l Biskeborn et al Oct. 28, 1947 2,607,010 Kock Aug. 12, 19522,67l,855 Van Atta Mar. 9, 1954 FOREIGN PATENTS 603,676 Great BritainOct. 25, 1946

